Referring to FIGS. 1 to 3 of the appended drawings, there is shown therein—respectively in plan view, in diametral section taken along the line II-II in FIG. 1 and in partially cut-away three-quarter perspective view from above—a mold bottom 1 intended to equip a mold (not shown) for the manufacture of thermoplastic material, such as PET, containers, in particular bottles, by drawing-blowing hot preforms, which mold is part of a molding device (not shown) that may include a plurality of molds, for example a rotary device of the carousel type in which the molds are distributed peripherally.
As shown in FIG. 4, which is a three-quarter isometric perspective view from below of the bottom of a container 2 molded using a mold bottom 1 as shown in FIGS. 1 to 3, the container 2 has a body 3 and a bottom 4 of the so-called petaloid type featuring a plurality of excrescences forming feet 5, generally from four to six in number (generally five in the standard practice, as shown). These feet are angularly distributed in equidistant fashion and extend approximately parallel to the axis 6 of the container. They are separated from each other by radiating valleys 7 with a convex curvilinear bottom 8 (which may extend in a circular arc). All the valleys 7 converge at the center of the bottom 4 of the container, which may include a plateau 10 projecting slightly outward. The bottom 4 of the container is connected to the body 3 of the container by a connecting area 9 that is substantially a circular-section cylinder.
The mold bottom 1 is arranged in a complementary fashion and includes, for molding said bottom 4 of the container 2, cavities 11 in a number equal to the number of feet 5 of the container bottom 4 (five in the example shown in FIGS. 1 to 3) and constitutes a petaloid mold bottom 1. These cavities are angularly distributed in equidistant fashion relative to the axis 12 of the mold bottom (mutual angular separation of 72° in this example) and are separated from each other by radiating ridges 13 (that is to say projecting portions, elongate in a substantially radial direction, of the mold bottom which form said valleys in the bottom of the molded container). Each ridge 13 has a peak 14 of concave (for example circular arc) extent.
When a container is fabricated by drawing-blowing a hot preform in a blowing mold equipped with the mold bottom 1, the preform 16 starts by being drawn longitudinally in mechanical fashion, by the action of a drawing rod 15 introduced into the preform 16 and movable axially; the end 17 of the drawing rod 15, bearing against the bottom 18 of the preform 16, drives the latter into contact with the mold bottom 1, as represented in chain-dotted line in FIG. 2.
In FIG. 2, it is seen clearly that, once the mechanical drawing has been completed, the distance D1 separating the bottom of a cavity 11 from the facing portion of the preform 16 is very much greater than the distance D2 separating a ridge 13 from the facing portion of the preform 16. This means that, during pneumatic blowing phases or pneumatic pre-blowing phases followed by pneumatic blowing that complements the mechanical drawing, the thermoplastic material of the bottom 18 of the preform 16 is subjected to longitudinal and/or radial drawing rates that are very different according to their location; this is the case in particular according to whether it is a question of conforming the extremity of the excrescences forming feet 5, for which the drawing rate both longitudinally and radially is a maximum, or a question of conforming the bottom 8 of the valleys 7 interleaved between the feet, for which the drawing rate is lower.
Remember that, as a general rule, molding by drawing-blowing includes, in addition to the mechanical drawing step, at least one step of blowing by means of a fluid, typically air, at a very high pressure (typically of the order of 25×105 Pa to 40×105 Pa). The blowing step is most often preceded by a pre-blowing step using a fluid at a relatively low pressure (typically of the order of 10 to 15×105 Pa). The inclusion or absence of the pre-blowing step, and likewise the sequencing of the various steps, depend on the characteristics of the container to be fabricated.
Remember also that, in most cases, the drawing step is initiated first, followed by the pre-blowing step which, in many cases, begins when drawing has not been completed; blowing then begins, sometimes when drawing has not been completed. In other cases, pre-blowing may be initiated before drawing. In certain configurations, blowing begins after drawing is completed. In other cases, the sequence may be as follows: complete drawing, followed by pre-blowing and blowing. In extreme cases, there is no pre-blowing operation and blowing begins before or after drawing is completed.
Other sequences may be envisaged. In the remainder of the description, the term “blowing” employed without other qualification will designate without distinction, and whatever the sequencing relative to mechanical drawing, a step of blowing only or the succession of a pre-blowing step and a blowing step.
Regardless of the sequencing of the operations, to obtain a correct conformation of the bottom 4 of the container as a whole, it is necessary for the thermoplastic material to be at a temperature producing the optimum softening for homogeneous drawing, whilst at the same time maintaining the temperature below the crystallization temperature. It is then certain that the softened material can be fed correctly into the areas that are subject to the highest drawing rates, and thus that a container is obtained with a bottom having a relatively constant thickness throughout despite its complex shape.
Now, the drawing rod 15, which is made of metal, is brought into contact sequentially with the bottoms of successive hot preforms and its temperature undoubtedly rises because of these repetitive contacts; however, these contacts being only intermittent, the rod remains at all times at an average temperature substantially lower than that of the hot preforms. To give a concrete idea of this, the preforms are typically heated to a temperature of the order of 100° C. whereas the average temperature of the drawing rod remains of the order of 85° C.
Because of this, it is clear that, in the area of contact of the drawing rod 15 with the thermoplastic material of the bottom 18 of the preforms 16, that material, on conceding heat to the metal rod that serves as thermal receiver, is cooled locally. As a result of this, the thermoplastic material, becoming cooler, at least locally, loses a portion of its drawing capacity. It must also be emphasized that, when pushed by the end 17 of the rod, the thermoplastic material of the bottom 18 of the preform is deformed and to some degree shrinks around the end 17 of the rod, which increases the area of contact. This increases the heat exchange cited above.
The consequence is that, because of the insufficient mobility of the thermoplastic material during blowing, and regardless of the time at which it begins after the drawing rod has touched the bottom of the preform, too great a quantity of material remains in the central area of the bottom of the container in corresponding relationship to the position of the bottom of the preform. In concrete terms, the bottom of the containers features a thickness that varies greatly according to the location; the material thickness is a maximum in a central ring around the central plateau 10 or button, whereas it is a minimum in the bottom of the feet because of the insufficient supply of material.
This drawback is relatively unimportant when manufacturing containers with standard bottoms that feature reliefs that are relatively little differentiated and does not affect the quality of manufacture of such containers. On the other hand, the drawback referred to hereinabove becomes entirely relevant when manufacturing containers with a petaloid bottom: the correct formation of the feet 5 of the bottom necessitates a high rate of drawing of the thermoplastic material whereas that material is precisely that which has been in contact with the drawing rod 15 and has suffered a reduction in temperature. However, for containers having normal or small capacities (that is to say having capacities not exceeding approximately 2 liters), this drawback is relatively unimportant and until now no compensation has been envisaged for this type of container. On the other hand, for containers of significantly greater capacity (for example 3 or 3.5 liter containers), the difference in dimensions between the feet and the valleys becomes much greater and such that it is no longer possible to produce petaloid bottoms of correct shape under the conditions presently applying to containers of smaller capacity.
In an attempt to remedy this drawback, at least in part, it has already been envisaged to reduce as much as possible the area of contact of the drawing rod with the thermoplastic material of the bottom of the hot preforms. To this end, it has been proposed to use shouldered drawing rods that feature an end part of reduced diameter connected to the rod body by a shoulder (or even by a plurality of successive staggered shoulders). With this arrangement, the thermoplastic material is in contact with the rod only at the rounded tip thereof and along the peripheral edge of the shoulder, which represents a smaller area of contact than with a conventional drawing rod.
However, this shouldered drawing rod has the drawback of being aggressive with respect to the preforms. The hot thermoplastic material, which is fragile and easily torn, can easily be pierced by the smaller diameter end of the rod because the thrust force communicated by the rod to the bottom of the preform is applied over a reduced area; what is more, the peripheral edge of the shoulder, if it comes into contact with the thermoplastic material, may mark it or even cause it to tear.